It is well known to apply conventional paper and plastic film labels to two-dimensional surfaces of blow molded plastic articles, such as shampoo bottles and beverage containers, after the part has been removed from the mold. To improve the efficiency of blow molding operations, transfer machines were developed which place labels on the blow mold itself before the plastic part is formed. When the plastic article is blow molded, the label either melts or adheres to the two-dimensional container surface.
U.S. Pat. No. 5,266,377, issued to Kinoshita et al., discloses a label and method for applying a label during in-mold molding. The label comprises a printed layer formed on a surface of a non-oriented plastic based film over which a membrane layer is overlapped. The label is inserted into a mold for in-mold labeling and is held in the proper position on the mold using vacuum suction or static electricity. Plastic is then injected into the mold and the membrane layer on the label bonds to the plastic article.
U.S. Pat. No. 5,223,315, issued to Katsura et al., discloses a label for two-dimensional surfaces consisting of a base of stretched film having a thermal shrinkage ratio greater than the mold shrinkage ratio of a plastic constituting the outer surface of a container. Using materials that satisfy these ratios prevents creasing and swelling of the label on containers with polyolefin outer surfaces.
U.S. Pat. No. 5,256,365, issued to Gordon et al., discloses a method of placing a label on mold cavities in a blow molding machine using an in-mold labeling system including first and second magazines with rotary members and vacuum assist.
U.S. Pat. No. 5,639,536, issued to Yamazaki et al., discloses the method of applying a transfer layer having projections and recesses finer than those on a molded part to produce a matte surface with good wear characteristics. The recesses on the molded part are up to approximately 300 microns deep and present therefore, essentially a two-dimensional surface.
The above-mentioned patents work satisfactorily for molded articles with thin walls, such as two-liter plastic beverage bottles and shampoo containers having wall thicknesses of approximately 0.015 to 0.060 inches. These articles retain relatively little heat in the walls compared to thick-walled molded articles, such as cases for power tools, which typically have wall thicknesses of approximately 0.055 to 0.250 inches. Thick-walled articles require a different label capable of withstanding the increased heat retained and subsequently released from the thick-walled sections. Moreover, the above-described background art is generally limited only to a two-dimensional surface and does not enhance the appearance of three-dimensional surfaces.
What is desired, therefore, is a label for in-mold labeling of plastic parts which is capable of being in a mold cavity therefore taking on the three-dimensional appearance of the relieved features or the entire molded product, and which is capable of being used in in-mold labeling operations on thick-walled molded parts which retain a substantial amount of heat in the thick walls. In another aspect, what is desired is a method of enhancing the three-dimensional appearance of a raised surface on a plastic part using an in-mold label.